Two Sides of the Same Coin: The Roles of KLF6 in Physiology and Pathophysiology

Abstract

The Krüppel-like factors (KLFs) family of proteins control several key biological processes that include proliferation, differentiation, metabolism, apoptosis and inflammation. Dysregulation of KLF functions have been shown to disrupt cellular homeostasis and contribute to disease development. KLF6 is a relevant example; a range of functional and expression assays suggested that the dysregulation of KLF6 contributes to the onset of cancer, inflammation-associated diseases as well as cardiovascular diseases. KLF6 expression is either suppressed or elevated depending on the disease, and this is largely due to alternative splicing events producing KLF6 isoforms with specialised functions. Hence, the aim of this review is to discuss the known aspects of KLF6 biology that covers the gene and protein architecture, gene regulation, post-translational modifications and functions of KLF6 in health and diseases. We put special emphasis on the equivocal roles of its full-length and spliced variants. We also deliberate on the therapeutic strategies of KLF6 and its associated signalling pathways. Finally, we provide compelling basic and clinical questions to enhance the knowledge and research on elucidating the roles of KLF6 in physiological and pathophysiological processes.

Keywords: KLFs; cancer; developmental process; gene regulation; inflammation; transcription factor.

Figure 1

KLF6 gene structure and the associated post-transcriptional modifications. (A) The KLF6 gene can be transcribed to produce 7 different transcripts, of which only KLF6-204, KLF6-206 and KLF6-207 are translated into proteins. (B) The primary KLF6 transcript, KLF6-206, contains four exons whereby germline mutation IVS 1-27 G > A can induce alternative splicing that gives rise to three additional KLF6 spliced variants: KLF6-SV1, KLF6-SV2 and KLF6-SV3.

Figure 2

A human KLF family phylogenetic tree generated using the Clustal Omega tool (https://www.ebi.ac.uk/Tools/msa/clustalo/). There are 17 members of the human KLF family, which can be further classified into three subgroups. KLF7 is the closest to KLF6 in the phylogeny.

Figure 3

The phylogenetic relationship among the five selected KLF6 orthologs from D. melanogaster, D. rerio, X. laevis, M. musculus and H. sapiens. The phylogram was generated using the Clustal Omega tool (https://www.ebi.ac.uk/Tools/msa/clustalo/).

Figure 4

KLF6 protein structure and the associated post-translational modifications. The KLF6 protein consists of three domains: (I) N-terminal acidic domain, (II) serine/threonine-rich central domain and (III) C-terminal DNA-binding domain. The protein structure is adapted from Andreoli et al. [17]. According to PhosphositePlus (https://www.phosphosite.org/), the KLF6 protein undergoes three types of post-translational modifications (PTMs). The sole ubiquitylation site for KLF6 is located at Lys66-Ub. Phosphorylation is the best-documented KLF6 PTM, encompassing T147-p, S150-p, S151-p, S171-p, S192-p and S233-p. Finally, four lysine acetylation sites have been documented for the C-terminal, i.e., S209-Ac, S213-Ac, S218-Ac and S228-Ac.

Figure 5

Diagram summarising the roles of KLF6 in normal physiological processes.

Figure 6

Dot plots representing the KLF6 expression RNA-Seq data in tumour and normal tissue samples from The Cancer Genome Atlas (TCGA) and The Genotype-Tissue Expression (GTEx) databases, respectively. KLF6 was found to be significantly upregulated and downregulated in acute myeloid leukaemia and uterine carcinoma, respectively (Log₂FC |2|; p-value < 0.05). The data were analysed and visualised using the GEPIA2 web application [80].

Figure 7

The frequency of KLF6 genetic alterations in twenty-six different cancer types that were molecularly characterised in the large scale TCGA project. These KLF6 genetic alterations frequency data were queried and extracted using cBioPortal (https://www.cbioportal.org/) [108,109]. The identified genetic alterations in KLF6 include mutation, amplification, deep deletion, fusion and multiple alterations. The “+” sign on top of each cancer type indicates that the mutation and copy number alteration (CNA) data are available for these cancer types.